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Trane Voyager Issues

I have been troubleshooting a "no heat" call on a Trane YCD061 rooftop unit and it seems I'm at a point where I need further assistance. This unit is equipped with a UCP and CTI board, both of which were replaced this past summer for problems with cooling. Now, during a call for heat from the programmable space thermostat, the unit will come on for about 10 seconds and then cycle off. Other times, when I jump out LTB 14 and LTB 5 to bypass the thermostat, nothing happens, but I do notice the green light on the IGN board is not lit. When I check the voltage at the TH+ terminal on the IGN board, I get 12 volts AC, not 24. I also get 12 volts AC at the J1-22 pin on the UCP but get 24 volts at the J1-21 pin (either side of the K6 contacts). It seems as thought I'm getting a voltage drop in the UCP board at the K6 contacts. But, when I unplug the cable at the IGN board and check the voltage at PPF3-2, I get 24 volts. I've checked both TC01 and TC02 and they're ok. If I take 24 volts right from the step-down transformer and apply it to TH+, the heat goes through a typical start sequence and lights off ok. The system runs ok when in the test mode.
Very frustrating. Any help would be appreciated!

Speaking for myself, I want you to get Pro membership if you are a working pro, and post in the closed forums.

It sounds as though an untrained person (or an unTraned person?) has been working on the unit, and things need to be checked in a methodical and professional manner, none of which can be imparted to you in an open forum on the site.

The first issue is jumping out terminals on a digital system. which terminals are which evade me as I'm not standing at it and looking at the diagram. It does have a set of "test terminals" at the top of that terminal strip that are meant to be "jumped out" by temporarily shorting or the installation of a resistor of the correctsize for you particular test.
If you connect ( short ) the test terminals together the system will cycle through cooling and heating stages and you can remove the jumper when the heating cycle starts. this will tell you if the basic control functions are working at that point.